This invention relates to an electric shaving apparatus having a housing and a shearing head system which is adapted to pivot relative to the housing about a pivot axis and comprises a shearing head frame, a shearing plane formed by at least one upper cutter and having arcuate extensions towards the longitudinal sides of the shearing head frame, and at least one under cutter driven by a motor and in cooperative relation with the upper cutter.
U.S. Des. Pat. No. 232,494 discloses a shaving apparatus having a pivoted twin-head shearing system whose pivot axis, which is determined by the pivot radius shown, lies below the line interconnecting the transition points of the effective shearing areas of the arcuate extensions to the shearing head frame, with the result that invariably only one shearing head will remain in engagement with the skin surface whilst the other will become disengaged from the skin surface, even when the two shearing heads are specifically made to engage the skin surface, in dependence on the respective direction of movement of the shearing heads over the skin.
From U.S. Pat. No. 2,099,537 a shearing head is known which is pivotally mounted on the hand grip of a shaving apparatus and has both planar shearing surfaces extending parallel to each other. In the shearing head disclosed in U.S. Pat. No. 2,099,537, the width of the shearing plane exposed to the contact pressure extends beyond the shearing range formed by upper and under cutters. It is bounded by two rollers serving to protect the skin which are associated with the planar shearing range outside thereof and extending parallel to its longitudinal direction. As disclosed in FIG. 8 of U.S. Pat. No. 2,099,537, the shearing plane has its pivot axis on the lever line connecting the center axes of the two rollers. On account of the distance between the shearing plane and the lever line extending parallel to the shearing plane and through the pivot axis, the forces of the contact pressure acting vertically on the shearing plane and the sliding force at which the shearing plane is moved over the skin surface combine with the sliding friction to produce a torque causing the shearing plane to engage the skin surface in the one direction of movement, while causing disengagement of the shearing plane from the skin surface in the opposite direction of movement. This disengagement effect occurs positively when the shaving apparatus comes in contact with the skin surface to be shaved through one of the rollers and is then moved over the skin.
The torque causing disengagement of the shearing plane from the skin surface becomes even greater if the pivot axis of the shearing plane is disposed below the lever line connecting the center axes of the two rollers, as disclosed in FIG. 4 of U.S. Pat. No. 2,099,537. The disengagement effect of a shearing plane as described in the foregoing with reference to FIG. 8 of U.S. Pat. 2,099,537 will also occur in the device of FIG. 3 of U.S. Pat. No. 2,526,153. In this device, the shearing plane is formed by the tangent plane connecting the peripheries of the two circular shearing heads. The effective shearing area of the two arcuate extensions covers the range from the respective transition points of the shearing head frame to the arcuate extensions and their tangent points into the shearing plane. In this device, the pivot axis for the lever arm connecting the two shearing heads is below the line interconnecting the transition points of the effective shearing areas of the arcuate extensions to the shearing head frame.
From German published and examined patent application DE-AS No. 1,048,509 a shaving apparatus is known having at least two adjacent shearing heads with plane shearing surface, the heads being movably carried in a shearing head frame stationarily mounted on a hand grip, with at least one of the shearing heads being movable relative to the other in a manner enabling the shearing heads to assume automatically positions, in response to the contact pressure exerted on the skin, in which their shearing surfaces are either in the same plane or at a more or less large relative angle. Such a control of the shearing head position in dependence on the contact pressures applied is based on the understanding that the hand grip with its shearing heads is pressed against the skin surface at almost right angles, allowing only a small amount of angular freedom for the hand grip to ensure that all shearing heads are in engagement with the skin. When the rolled edges provided in the longitudinal direction are in contact with the skin, the shearing surface of this known device is excluded from such contact. The pivot axis in each of the embodiments of DE-AS No. 1,048,509 is below the line interconnecting the transition points of the effective shaving area of the arcuate extensions to the shearing head frame.